Chapter 7: Alkenes and Alkynes

• Hydrocarbons Containing Double and Triple Bonds

• Unsaturated Compounds (Less than Maximum H Atoms)

• Alkenes also Referred to as Olefins

• Properties Similar to those of Corresponding Alkanes

• Slightly Soluble in Water

• Dissolve Readily in Nonpolar or Low Polarity Solvents

• Densities of Alkenes and Alkynes Less than Water

Isomerism: Cis/Trans

C

C

Cl H

Cl H

C

C

H Cl

Cl H

Cis or (Z) Trans or (E)

• Same Molecular Formula (C2Cl2H2) and Connectivity

• Different Structures Double Bonds Don’t Rotate

• For Tri/Tetra Substituted Alkenes; Use (E), (Z) Labels

Alkenes: Relative Stability

Tetrasubstituted Trisubstituted Geminal Disubstituted Trans Disubstituted

Cis Disubstituted Monosubstituted Unsubstituted

> > >

> >

• Higher Alkyl Substitution = Higher Alkene Stability

• Note Stability Trends of Disubstituted Alkenes

• Can Also Observe Cyclic Alkenes

Alkenes: Cyclic Structures

• Note all of These are Cis Alkenes

• Can Observe Trans Cycloalkenes; z.b. trans-Cycloctene

• trans-Cycloheptene Observable Spectroscopically; Can’t Isolate

HC CH

CH2

H2C

H2C

HC

HCCH

CH

CH

HC

HC

HC CH2

CH2HC

HCCH2

HC

HCCH2

CH2

CH2

H2C

Cyclopropene Cyclobutene Cyclopentene

Cyclohexene Cyclohexatriene (Benzene)

Alkenes: Synthesis via Elimination

• Dehydrohalogenation; E2 Elimination Reaction

• E2 Reactions Preferable Over E1 (Rearrangement; SN1 Products)

• Usually Heat These Reactions (Heat Favors Elimination)

H

Br

H

H

H

H

C2H5ONaH

HH

H

H

Br

H

H

H

HO

H

HH

H

Alkenes: Zaitsev’s Rule

• If Multiple Possible Products; Most Stable (Substituted) Forms

• More Substituted: Product and Transition State Lower in Energy

H

Br

CH3

H3C

H

CH2

C2H5ONa

CH3H3C

H

H

H

Br

CH3

H3C

H

CH3

C2H5ONaCH3

CH3H3C

H

31%

69%

Alkenes: Forming the Least Substituted

• Bulky Base Favors Least Substituted Product

• Due to Steric Crowding in Transition State (2° Hydrogens)

H

Br

CH3

H3C

H

CH2

CH3H3C

H

H

H

Br

CH3

H3C

H

CH3 CH3

CH3H3C

H

72.5%

27.5%

OK

OK

Alkenes: The Transition State in E2

• Orientation Allows Proper Orbital Overlap in New Bond

• Syn Coplanar Transition State only in Certain Rigid Systems

• Anti: Staggered; Syn: Eclipsed Anti TS is Favored

H

Br

H

H

H

HO

Anti Coplanar Conformation(Hydrogen and Leaving Group)

Alkenes: E2 Reactions of Cyclohexanes

• Anti Transition State Attainable w/ Axial H and Leaving Group

• Axial/Equatorial and Equatorial/Equatorial Improper Combos

• Let’s Look at Higher Substituted Cyclohexanes

Cl

H

EtO

Alkenes: E2 Reactions of Cyclohexanes

• Multiple H’s Axial to Leaving Group Multiple Products

• Zaitsev’s Rule Governs Product Formation

• What if NO Anti Coplanar Arrangement in Stable Conformer??

Cl

HiPr

H

+

iPr iPr

Me Me

22% 78%(Zaitsev's Rule)

EtOEtO

Alkenes: E2 Reactions of Cyclohexanes

• All Groups Equatorial in Most Stable Conformation

• Chair Flip Form has Proper Alignment

• Reaction Proceeds Through High Energy Conformation

• Only ONE Possible Elimination Product In This Case

MeiPr

iPr

Me

100%

Cl

Cl

iPr

Me

H

EtO

Alkenes: Acid Catalyzed Dehydration

• Have to Pound 1° Alcohols to Dehydrate w/ Acid

• 2° Alcohols Easier, Can Use Milder Conditions

H

H

H

H

OH

Hconcd H2SO4

180 oC

H

H H

H

+ H2O

OH

H

85% H3PO4

165-170 oC+ H2O

Alkenes: Acid Catalyzed Dehydration

• 3° Alcohols Exceptionally Easy to Dehydrate

• Can Use Dilute Acid, Lower Temperatures

• Relative Ease of Reaction:

3° > 2° > 1°

20% H2SO4

85 oC+ H2OH3C OH

CH3

CH2

H

CH2

CH3H3C

Alkenes: Acid Catalyzed Dehydration

• E1 Elimination Reaction Mechanism (Explains Ease)

H3C OH

CH3

CH2

H

H+H3C OH2

CH3

CH2

H

CH3

CH3CH2

H+ H2O

CH2

CH3H3C

-H+

-H2O

Base

Alkenes: Acid Catalyzed Dehydration

• 3° Alcohols Easiest to Dehydrate via E1; 1° Hardest

• Recall Carbocation Stablility: 3° > 2° > 1°

• Relative Transition State Stability Related to Carbocation

• Why Are More Substituted Carbocations More Stable??

HYPERCONJUGATION (Donating Power of Alkyls)

• 1° Carbocation Instablility Dehydration of These is E2

Alkenes: 1° Alcohol Dehydration (E2)

H3C

CH3

H H

H

OH H A H3C

CH3

H H

H

OH2

A

H3C

H3C H

H

+ H2O + H-A

• Step One Fast

• Step Two Slow (RDS)

• Proceeds via E2 Due to Primary Carbocation Instability

• Sulfuric and Phosphoric Acids Are Commonly Used Acids

Carbocation Rearrangements

H3C

CH3

CH3

H

OH

CH3

85% H3PO4

Heat

CH3

CH3H3C

H3C

H

H CH3

CH(CH3)2

+

Major Minor

H3C

CH3

CH3

H

OH2

CH3 H3C

CH3

CH3

H

CH3

• A Priori One Expects the Minor Dehydration Product

• This Dehydration Product is NOT Observed Major Product

Carbocation Rearrangements (2)

• Methanide Migration Results in More Stable 3° Carbocation

• This Carbocation Gives Rise to Observed Major Product

• Can Also Observe HYDRIDE (H-) Shifts More Stable C+

H3C

CH3

CH3

H

CH3

Secondary Carbocation

H3C

CH3 H

CH3

CH3

Tertiary Carbocation

Methanide

Migration

H3C

CH3 H

CH3

CH

Transition State

Alkyne Synthesis: Dehydrohalogenation

H

R

BrBr

H

R R R2 eq. NaNH2

• Compounds w/ Halogens on Adjacent Carbons:

VICINAL Dihalides (Above Cmpd: Vicinal Dibromide)

• Entails Consecutive E2 Elimination Reactions

• NaNH2 Strong Enough to Effect Both Eliminations in 1 Pot

• Need 3 Equivalents NaNH2 for Terminal Alkynes

Reactions: Alkylation of Terminal Alkynes

• NaNH2 (-NH2) to Deprotonate Alkyne (Acid/Base Reaction)

• Anion Reacts with Alkyl Halide (Bromide); Displaces Halide

• Alkyl Group Added to Alkyne

• Alkyl Halide Must be 1° or Me; No Branching at 2nd () Carbon

H3C HNaNH2

NH3H3C

CH3BrH3C CH3

H3C HNaNH2

NH3H3C

EtBrH3C Et

Reactions: Alkylation of Terminal Alkynes

• SN2 Substitution Reactions on 1° Halides

• E2 Eliminations Occur on Reactions w/ 2°, 3° Halides

• Steric Problem; Proton More Accessible thanElectrophilic Carbon Atom

H3CH C

C

H3C

Br

HCH3

H

H3C H

+H3C

CH3

Alkenes: Hydrogenation Reactions

H2

Pt, Pd, or Ni (catalyst)Solvent, Pressure

Alkene Alkane

• Catalytic Hydrogenation is a SYN Addition of H2

• SYN Addition: Both Atoms Add to Same Side (Face) of Bond

• Catalyst: Lowers Transition State Energy (Activation Energy)

Alkynes: Hydrogenation Reactions

2H2

Pt (catalyst)Solvent, Pressure

AlkaneAlkyne• Platinum Catalysts Allow Double Addition of H2 On Alkyne

• Can Also Hydrogenate Once to Generate Alkenes

• Cis and Trans (E and Z) Stereoisomers are Possible

• Can Control Stereochemistry with Catalyst Selection

Alkynes: Hydrogenation to Alkenes

H H

H2/Ni2B

97%

R R

H

RR

H

H2, Pd/CaCO3

Quinoline

• SYN Additions to Alkynes (Result in cis/Z Alkenes)

• Reaction Takes Place on Surface of Catalyst

• Examples of a HETEROGENEOUS Catalyst System

Alkynes: Hydrogenation to Alkenes

(1) Li, C2H5NH2

(2) NH4ClH

H

• Dissolving Metal Reduction Reaction

• ANTI Addition of H2 to Alkyne E (trans) Stereoisomer

• Ethylamine or Ammonia can be used for Reaction

More On Unsaturation Numbers

• Unsaturation Number (r + ) Index of Rings and Multiple Bonds

• r + = C - ½ H + ½ N - ½ Halogen + 1

• Useful When Generating Structures from Molecular Formula

• Also Called Degree of Hydrogen Deficiency; Number of DoubleBond Equivalencies

• Often Combined with Spectroscopic Data when MakingUnknown Structure Determinations

Chapter 7: Key Concepts

• E2 Eliminations w/ Large and Small Bases

• E1 Elimination Reactions

• Zaitsev’s Rule

• Carbocation Rearrangement

• Dehydration and Dehydrohalogenation Reactions

• Synthesis of Alkynes

• Hydrogenation Reactions (Alkynes to E/Z Alkenes)

• Unsaturation Numbers; Utility in Structure Determination